If you want to use a pond filter for your 125 you should look up what it will take to run each filter as most do not come with a pump,, do a google search on ( gravity pond filters) this might get you were you are tyring to go,, th
my friend uses those pond filters for his 244G tanks he has a sump with the pond filter in it and the over flow boxes drain into the sump then the water goes thru the pond filter and is pumped back into the tank. works fine for hm and his tanks are clear and always clean looking.
i have the 850g home depot sells for 79.99 on my 120 wide with a small cascade canister (up to 80 gal).
I will be removing the cascade soon to see what happens I use a 600gph pump with 11 foot max head that uses 34 watts.
The filter has 3 gallons of capcity and comes with some bioballs, I'm going to load it with ceramix rings and maybe bath poofs as they are working in my cascade.